Head-mounted display apparatus employing one or more Fresnel lenses

ABSTRACT

Head-mounted displays ( 100 ) are disclosed which include a frame ( 107 ), an image display system ( 110 ) supported by the frame ( 107 ), and a Fresnel lens system ( 115 ) supported by the frame ( 107 ). The HMD ( 100 ) can employ a reflective optical surface, e.g., a free-space, ultra-wide angle, reflective optical surface (a FS/UWA/RO surface) ( 120 ), supported by the frame ( 107 ), with the Fresnel lens system ( 115 ) being located between the image display system ( 110 ) and the reflective optical surface ( 120 ). The Fresnel lens system ( 115 ) can include at least one curved Fresnel lens element ( 820 ). Fresnel lens elements ( 30 ) for use in HMDs are also disclosed which have facets ( 31 ) separated by edges ( 32 ) which lie along radial lines ( 33 ) which during use of the HMD pass through a center of rotation ( 34 ) of a nominal user&#39;s eye ( 35 ) or through the center of the eye&#39;s lens ( 36 ) or are normal to the surface of the eye&#39;s cornea.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/211,365, filed on Aug. 17, 2011, entitled “HEAD-MOUNTEDDISPLAY APPARATUS EMPLOYING ONE OR MORE FRESNEL LENSES,” which claimsthe benefit of U.S. Provisional Patent Application No. 61/405,440, filedOct. 21, 2010, entitled “HEAD-MOUNTED DISPLAY,” U.S. Provisional PatentApplication No. 61/417,325, filed Nov. 26, 2010, entitled“CURVED-STACKED FRESNEL ARCHITECTURE,” U.S. Provisional PatentApplication No. 61/417,326, filed Nov. 26, 2010, entitled “CURVED-BEAMSPLITTER ARCHITECTURE.” U.S. Provisional Patent Application No.61/417,327, filed Nov. 26, 2010, entitled “COMBINED ARCHITECTURE OFFRESNEL LENSE AND FLAT BEAM SPLITTER,” U.S. Provisional PatentApplication No. 61/417,328, filed Nov. 26, 2010, entitled “COMBINEDARCHITECTURE OF FRESNEL LENSE AND CURVED BEAM SPLITTER,” and U.S.Provisional Patent Application No. 61/427,530, filed Dec. 28, 2010,entitled “CURVED MIRROR FOR HEAD MOUNTED DISPLAY,” the disclosures ofwhich are hereby incorporated herein by reference in their entireties.

FIELD

This disclosure relates to head-mounted display apparatus employing oneor more Fresnel lenses. In certain embodiments, the apparatus alsoemploys one or more reflective optical surfaces, e.g., one or more freespace, ultra-wide angle, reflective optical surfaces (hereinafterabbreviated as “FS/UWA/RO surfaces”). In certain embodiments, theoverall optical system is a non-pupil forming system, i.e., thecontrolling aperture (aperture stop) of the entire system is the pupilof the user's eye.

The one or more Fresnel lenses and, when used, the one or morereflective surfaces (e.g., the one or more FS/UWA/RO surfaces) areemployed to display imagery from a light-emitting display system held inclose proximity to a user's eye.

BACKGROUND

A head-mounted display such as a helmet-mounted display oreyeglass-mounted display (abbreviated herein as a “HMD”) is a displaydevice worn on the head of an individual that has one or more smalldisplay devices located near one eye or, more commonly, both eyes of theuser.

Some HMDs display only simulated (computer-generated) images, as opposedto real-world images, and accordingly are often referred to as “virtualreality” or immersive HMDs. Other HMDs superimpose (combine) a simulatedimage upon a non-simulated, real-world image. The combination ofnon-simulated and simulated images allows the HMD user to view the worldthrough, for example, a visor or eyepiece on which additional datarelevant to the task to be performed is superimposed onto the forwardfield of view (FOV) of the user. This superposition is sometimesreferred to as “augmented reality” or “mixed reality.”

Combining a non-simulated, real-world view with a simulated image can beachieved using a partially-reflective/partially-transmissive opticalsurface (a “beam splitter”) where the surface's reflectivity is used todisplay the simulated image as a virtual image (in the optical sense)and the surface's transmissivity is used to allow the user to view thereal world directly (referred to as an “optical see-through system”).Combining a real-world view with a simulated image can also be doneelectronically by accepting video of a real world view from a camera andmixing it electronically with a simulated image using a combiner(referred to as a “video see-through system”). The combined image canthen be presented to the user as a virtual image (in the optical sense)by means of a reflective optical surface, which in this case need nothave transmissive properties.

From the foregoing, it can be seen that reflective optical surfaces canbe used in HMDs which provide the user with: (i) a combination of asimulated image and a non-simulated, real world image, (ii) acombination of a simulated image and a video image of the real world, or(iii) purely simulated images. (The last case is often referred to as an“immersive” system.) In each of these cases, the reflective opticalsurface produces a virtual image (in the optical sense) that is viewedby the user. Historically, such reflective optical surfaces have beenpart of optical systems whose exit pupils have substantially limited notonly the dynamic field of view available to the user, but also thestatic field of view. Specifically, to see the image produced by theoptical system, the user needed to align his/her eye with the opticalsystem's exit pupil and keep it so aligned, and even then, the imagevisible to the user would not cover the user's entire full static fieldof view, i.e., the prior optical systems used in HMDs that have employedreflective optical surfaces have been part of pupil-forming systems andthus have been exit-pupil-limited.

The reason the systems have been so limited is the fundamental fact thatthe human field of view is remarkably large. Thus, the static field ofview of a human eye, including both the eye's foveal and peripheralvision, is on the order of ˜150° in the horizontal direction and on theorder of ˜130° in the vertical direction. (For the purposes of thisdisclosure, 150 degrees will be used as the straight ahead static fieldof view of a nominal human eye.) Well-corrected optical systems havingexit pupils capable of accommodating such a large static field of vieware few and far between, and when they exist, they are expensive andbulky.

Moreover, the operational field of view of the human eye (dynamic fieldof view) is even larger since the eye can rotate about its center ofrotation, i.e., the human brain can aim the human eye'sfoveal+peripheral field of view in different directions by changing theeye's direction of gaze. For a nominal eye, the vertical range of motionis on the order of ˜40° up and ˜60° down and the horizontal range ofmotion is on the order of ±˜50° from straight ahead. For an exit pupilof the size produced by the types of optical systems previously used inHMDs, even a small rotation of the eye would substantially reduce whatoverlap there was between the eye's static field of view and the exitpupil and larger rotations would make the image disappear completely.Although theoretically possible, an exit pupil that would move insynchrony with the user's eye is impractical and would be prohibitivelyexpensive.

In view of these properties of the human eye, there are three fields ofview which are relevant in terms of providing an optical system whichallows a user to view an image generated by an image display system inthe same manner as he/she would view the natural world. The smallest ofthe three fields of view is that defined by the user's ability to rotatehis/her eye and thus scan his/her fovea over the outside world. Themaximum rotation is on the order of ±50° from straight ahead, so thisfield of view (the foveal dynamic field of view) is approximately 1000.The middle of the three fields of view is the straight ahead staticfield of view and includes both the user's foveal and peripheral vision.As discussed above, this field of view (the foveal+peripheral staticfield of view) is on the order of 150°. The largest of the three fieldsof view is that defined by the user's ability to rotate his/her eye andthus scan his/her foveal plus his/her peripheral vision over the outsideworld. Based on a maximum rotation on the order of ±50° and afoveal+peripheral static field of view on the order of 150°, thislargest field of view (the foveal+peripheral dynamic field of view) ison the order of 200°. This increasing scale of fields of view from atleast 100 degrees to at least 150 degrees and then to at least 200degrees provides corresponding benefits to the user in terms of his/herability to view images generated by an image display system in anintuitive and natural manner.

In order for the human eye to focus easily on a display that is within10 inches of the eye, a form of collimation needs to be applied to thelight rays emanating from the display. The collimation serves to makethe light rays appear as if they originate from a distance greater thanthe actual distance between the eye and the display. The greaterapparent distance, in turn, allows the eye to readily focus on an imageof the display. Some head-mounted displays use multiple mirrors orprisms in an attempt to collimate light from the display. The use ofmultiple mirrors or prisms adds bulk and weight, making suchhead-mounted displays more cumbersome and heavier than desired.

There thus exists a need for head-mounted displays that are compatiblewith the focusing ability as well as with at least the foveal dynamicfield of view of the human eye. The present disclosure is directed tothese needs and provides head-mounted displays that produce collimated(or substantially collimated) light over a wide field of view.

Definitions

In the remainder of this disclosure and in the claims, the phrase“virtual image” is used in its optical sense, i.e., a virtual image isan image that is perceived to be coming from a particular place where infact the light being perceived does not originate at that place.

Throughout this disclosure, the following phrases/terms shall have thefollowing meanings/scope:

-   -   (1) The phrase “a reflective optical surface” (also referred to        herein as a “reflective surface”) shall include surfaces that        are only reflective as well as surfaces that are both reflective        and transmissive. In either case, the reflectivity can be only        partial, i.e., part of the incident light can be transmitted        through the surface. Likewise, when the surface is both        reflective and transmissive, the reflectivity and/or the        transmissivity can be partial. As discussed below, a single        reflective optical surface can be used for both eyes or each eye        can have its own individual reflective optical surface. Other        variations include using multiple reflective optical surfaces        for either both eyes or individually for each eye. Mix and match        combinations can also be used, e.g., a single reflective optical        surface can be used for one eye and multiple reflective optical        surfaces for the other eye. As a further alternative, one or        multiple reflective optical surfaces can be provided for only        one of the user's eyes. The claims set forth below are intended        to cover these and other applications of the reflective optical        surfaces disclosed herein. In particular, each claim that calls        for a reflective optical surface is intended to cover        head-mounted display apparatus that includes one or more        reflective optical surfaces of the type specified.    -   (2) The phrase “an image display system having at least one        light-emitting surface” is used generally to include any display        system having a surface which emits light whether by        transmission of light through the surface, generation of light        at the surface (e.g., by an array of LEDs), reflection off of        the surface of light from another source, or the like. The image        display system can employ one or multiple image display devices,        e.g., one or multiple LED and/or LCD arrays. As with reflective        optical surfaces, a given head-mounted display apparatus can        incorporate one or more image display systems for one or both of        the user's eyes. Again, each of the claims set forth below that        calls for an image display system is intended to cover        head-mounted display apparatus that includes one or more image        display systems of the type specified.    -   (3) The phrase “binocular viewer” means an apparatus that        includes at least one separate optical element (e.g., one        display device and/or one reflective optical surface) for each        eye.    -   (4) The phrase “field of view” and its abbreviation FOV refer to        the “apparent” field of view in image (eye) space as opposed to        the “real” field of view in object (i.e., display) space.

SUMMARY

In accordance with a first aspect, a head-mounted display apparatus(100) is disclosed which includes:

(I) a frame (107) adapted to be mounted on a user's head (105);

(II) an image display system (110) supported by the frame (107) (e.g.,the frame supports the image system device at a fixed location which,during use of the HMD, is outside of the user's field of view);

(III) a reflective optical surface (120) supported by the frame (107),the reflective optical surface (120) being a continuous surface that isnot rotationally symmetric about any coordinate axis of athree-dimensional Cartesian coordinate system (e.g., the reflectiveoptical surface can be a free-space, ultra-wide angle, reflectiveoptical surface (120) which is not rotationally symmetric (is not asurface of revolution) about the x, y, or z axes of a three-dimensionalCartesian coordinate system having an arbitrary origin); and

(IV) a Fresnel lens system (115) supported by the frame (107), theFresnel lens system (115) being located between the image display system(110) and the reflective optical surface (120);

wherein:

(a) the image display system (110) includes at least one light-emittingsurface (81);

(b) during use, the reflective optical surface (120) and the Fresnellens system (115) produce spatially-separated virtual images ofspatially-separated portions of the at least one light-emitting surface(81), at least one of the spatially-separated virtual images beingangularly separated from at least one other of the spatially-separatedvirtual images by at least 100 degrees (in some embodiments, at least150 degrees and, in other embodiments, at least 200 degrees), theangular separation being measured from a center of rotation (72) of anominal user's eye (71); and

(c) during use, at least one point of the reflective optical surface(120) is angularly separated from at least one other point of thereflective optical surface (120) by at least 100 degrees (in someembodiments, at least 150 degrees and, in other embodiments, at least200 degrees), the angular separation being measured from the center ofrotation of a nominal user's eye.

For this aspect, during use, the at least one of the spatially-separatedvirtual images can be located along a direction of gaze which passesthrough the at least one point of the reflective optical surface and theat least one other of the spatially-separated virtual images is locatedalong a direction of gaze which passes through the at least one otherpoint of the reflective optical surface.

In accordance with a second aspect, a head-mounted display apparatus(100) is disclosed which includes:

(I) a frame (107) adapted to be mounted on a user's head (105);

(II) an image display system (110) supported by the frame (107) (e.g.,the frame supports the image display system at a fixed location which,during use of the HMD, is outside of the user's field of view);

(III) a free-space, ultra-wide angle, reflective optical surface (120)supported by the frame (107); and

(IV) a Fresnel lens system (115) supported by the frame (107), theFresnel lens system (115) being located between the image display system(110) and the free-space, ultra-wide angle, reflective optical surface(120);

wherein:

(a) the image display system (110) includes at least one light-emittingsurface (81);

(b) during use, the free-space, ultra-wide angle, reflective opticalsurface (120) and the Fresnel lens system (115) producespatially-separated virtual images of spatially-separated portions ofthe at least one light-emitting surface (81), at least one of thespatially-separated virtual images being angularly separated from atleast one other of the spatially-separated virtual images by at least100 degrees (in some embodiments, at least 150 degrees and, in otherembodiments, at least 200 degrees), the angular separation beingmeasured from a center of rotation (72) of a nominal user's eye (71).

In accordance with a third aspect, a head-mounted display apparatus(100) is disclosed that includes:

(I) a frame (107) adapted to be mounted on a user's head (105);

(II) an image display system (110) supported by the frame (107);

(III) a reflective surface (120) supported by the frame (107); and

(IV) a Fresnel lens system (115) supported by the frame (107), theFresnel lens system (115) being located between the image display system(110) and the reflective optical surface (120);

wherein the Fresnel lens system (115) includes at least one Fresnel lenselement that is curved.

In accordance with a fourth aspect, a head-mounted display apparatus(100) is disclosed that includes:

(I) a frame (107) adapted to be mounted on a user's head (105);

(II) an image display system (110) supported by the frame (107); and

(III) a Fresnel lens system (115) supported by the frame (107);

wherein:

during use, the Fresnel lens system (115) is located between the imagedisplay system (110) and a nominal user's eye; and

the Fresnel lens system (115) includes at least one Fresnel lens element(30) having a plurality of facets (31) that are separated from anotherby edges (32) wherein during use of the head-mounted display apparatus,at least some of the edges (32) lie along radial lines that (i) passthrough a center of rotation (34) of a nominal user's eye (35), or (ii)pass through the center of a nominal user's natural lens (i.e., thenominal user's crystalline lens), or (iii) are normal to the surface ofa nominal user's cornea.

In certain embodiments of the above aspects of the disclosure, aseparate Fresnel lens system, a separate image display system, and/or aseparate reflective surface (when used) is employed for each of theuser's eyes. In other embodiments, the reflective optical surface (whenused) contributes to the collimation (or substantial collimation) of thelight from the image display system provided by the Fresnel lens system,such contribution to the collimation (or substantial collimation) beingachieved through the surface's local radii of curvature.

In various embodiments, the HMD apparatus may be a binocularnon-pupil-forming system in which the eye is free to move about itsrolling center throughout its normally obtainable angular extentswithout being constrained to look through an external pupil. Prior HMDdevices have alleged that they have or can provide a wide field of view,but these devices have included an external pupil that the eye must lookthrough. Although there is a wide amount of information provided to theeye, if the eye turns the information is gone. This is the fundamentalproblem with pupil-forming systems which is avoided in embodiments ofthe present disclosure which employ reflective surfaces and, inparticular, FS/UWA/RO surfaces.

The reference numbers used in the above summaries of the aspects of theinvention (which reference numbers are representative and notall-inclusive or exhaustive) are only for the convenience of the readerand are not intended to and should not be interpreted as limiting thescope of the invention. More generally, it is to be understood that boththe foregoing general description and the following detailed descriptionare merely exemplary of the invention and are intended to provide anoverview or framework for understanding the nature and character of theinvention.

Additional features and advantages of the invention are set forth in thedetailed description which follows, and in part will be readily apparentto those skilled in the art from that description or recognized bypracticing the invention as exemplified by the description herein. Theaccompanying drawings are included to provide a further understanding ofthe invention, and are incorporated in and constitute a part of thisspecification. It is to be understood that the various features of theinvention disclosed in this specification and in the drawings can beused in any and all combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view representation of a head-mounted display apparatusaccording to an example embodiment.

FIG. 2 is a front view representation of the head-mounted displayapparatus of FIG. 1.

FIG. 3 is a schematic cross-sectional view of a Fresnel lens elementhaving facets whose edges pass through the center of rotation of auser's eye according to an example embodiment.

FIG. 4 illustrates an optical system for a head-mounted displayapparatus that includes a Fresnel lens system and a curved reflectiveoptical surface according to an example embodiment.

FIG. 5 is a top view of a head-mounted display apparatus illustratingthe use of two curved reflective optical surfaces corresponding to thetwo eyes of a user according to an example embodiment.

FIG. 6 is a schematic diagram illustrating a static field of view of anominal human eye for a straight ahead direction of gaze.

FIG. 7 is a schematic diagram illustrating the interaction between thestatic field of view of FIG. 6 with a FS/UWA/RO surface according to anexample embodiment. The arrows in FIG. 7 illustrate directions of lightpropagation.

FIG. 8 is a ray diagram illustrating a light path from a given pixel ona display as it is reflected toward an eye according to an exampleembodiment.

FIG. 9 is a ray diagram illustrating light paths from two pixels on adisplay as they are reflected toward an eye according to an exampleembodiment.

FIG. 10 is a diagram illustrating variables used in selecting thedirection of the local normal of a reflector according to an exampleembodiment.

FIG. 11 is a representation of a curved reflector along with light pathsaccording to an example embodiment.

FIG. 12 is a block diagram of a side view of an augmented-realityhead-mounted display apparatus having a Fresnel lens system according toan example embodiment.

FIG. 13 is a ray diagram illustrating light rays in an augmented-realityhead-mounted display apparatus of the type shown in FIG. 12.

FIG. 14 is a ray diagram illustrating display and external light rays inthe augmented-reality head-mounted display apparatus of FIG. 13.

FIG. 15 is a block diagram of a side view of an immersive head-mounteddisplay apparatus having a Fresnel lens system according to an exampleembodiment.

FIG. 16 is a block diagram of a top view of an immersive head-mounteddisplay apparatus having a Fresnel lens system according to an exampleembodiment.

FIG. 17 is a ray diagram illustrating light rays in an immersivehead-mounted display apparatus of type shown in FIGS. 15 and 16.

FIG. 18 is ray diagram illustrating light rays entering an eye of a useraccording to an example embodiment.

FIG. 19 is a schematic diagram illustrating geometry for calculating alocal normal to a reflective surface according to an example embodiment.

DETAILED DESCRIPTION I. Introduction

As discussed above, the present disclosure relates to HMDs which providea user with a collimated (or substantially collimated) image through theuse of a Fresnel lens system, which may be a curved Fresnel lens system(see below). The Fresnel lens system may be the sole source ofcollimation in the optical system or, in embodiments that employ curvedreflective optical surface, e.g., a FS/UWA/RO surface, the Fresnel lenssystem's collimation may be combined with collimation contributed by thecurved reflective optical surface.

The following discussion begins with a description of embodiments thatemploy a FS/UWA/RO surface (Section II) and then proceeds to adiscussion of Fresnel lens systems for use with those embodiments aswell as other embodiments disclosed herein (Section III). Section IIIalso includes a discussion of the design process for a FS/UWA/RO surfacethat is used in an optical system that includes a Fresnel lens system.Following Section (III), embodiments that employ a reflective opticalsurface that is not a FS/UWA/RO surface and a curved Fresnel lenssystems are discussed (Section IV), followed by embodiments in which animage display system is viewed directly through a curved Fresnel lenssystem without the use of a reflective surface (Section V). Finally, ageneral discussion applicable to the various embodiments disclosedherein is presented (Section VI).

It should be understood that the discussions of the various componentsof HMDs that appear in particular sections of the presentation are notlimited to the embodiments of that section, but are generally applicableto all of the embodiments disclosed herein. As one example, thedescription of the types of image display systems that may be used in aHMD is applicable to the Section I embodiments (where the descriptionappears), as well as to the Sections IV and V embodiments.

II. HMDs that Employ FS/UWA/RO Surfaces

FIGS. 1 and 2 are, respectively, a side view and a front view of ahead-mounted display apparatus 100 shown being worn by a user 105. Thehead-mounted display apparatus employs a FS/UWA/RO surface 120.

In one embodiment, the head-mounted display apparatus 100 can be, forexample, an optical see-through, augmented reality, binocular viewer.Because an optical see-through, augmented reality, binocular viewer istypically the most complex form of a HMD, the present disclosure willprimarily discuss embodiments of this type, it being understood that theprinciples discussed herein are equally applicable to opticalsee-through, augmented reality, monocular viewers, video see-through,augmented reality, binocular and monocular viewers, and binocular andmonocular “virtual reality” systems.

As shown in FIGS. 1 and 2, the head-mounted display apparatus 100includes a frame 107 adapted to be worn by the user and supported by theuser's nose and ears in a manner similar to that in which eyeglasses areworn. In the embodiment of FIGS. 1-2, as well as in the otherembodiments disclosed herein, the head-mounted display apparatus mayhave a variety of configurations and can, for example, resembleconventional goggles, glasses, helmets, and the like. In someembodiments, a strap may be used to hold the HMD's frame in a fixedposition with respect to the eyes of the user. In general terms, theoutside surface of the HMD package can assume any form that holds theoptical system in the required orientation with respect to the HMD'sdisplay(s) and the user's eyes.

The head-mounted display apparatus 100 includes at least one imagedisplay system 110 and, as shown in FIGS. 1 and 2, a free space,ultra-wide angle, reflective optical surface 120, i.e., a FS/UWA/ROsurface 120, which by necessity is curved. Surface 120 can be purelyreflective or can have both reflective and transmissive properties, inwhich case, it can be thought of as a type of “beam splitter.”

Surface 120 is referred to herein as a “free space” surface because itslocal spatial positions, local surface curvatures, and local surfaceorientations are not tied to a particular substrate, such as the x-yplane, but rather, during the surface's design, are determined usingfundamental optical principles (e.g., the Fermat and Hero least timeprinciple) applied in three dimensional space. Surface 120 is referredto as an “ultra-wide angle” surface because, during use, at a minimum,it does not limit the dynamic foveal field of view of a nominal user'seye. As such, depending on the optical properties of the Fresnel lenssystem with which the FS/UWA/RO surface is used, the overall opticalsystem of the HMD can be non-pupil forming, i.e., unlike conventionaloptical systems that have an exit pupil which limits the user's field ofview, the operative pupil for various embodiments of the optical systemsdisclosed herein will be the entrance pupil of the user's eye as opposedto one associated with the external optical system. Concomitantly, forthese embodiments, the field of view provided to the user will be muchgreater than conventional optical systems where even a smallmisalignment of the user's eye with the exit pupil of the externaloptical system can substantially reduce the information contentavailable to the user and a larger misalignment can cause the entireimage to disappear.

FS/UWA/RO surface 120 may completely surround one or both eyes, as wellas the at least one image display system 110. In particular, the surfacecan curve around the sides of the eyes and toward the sides of the faceso as to expand the available horizontal field of view. In oneembodiment, the FS/UWA/RO surface 120 may extend up to 180° or more(e.g., more than 200°), as best seen in FIG. 5 discussed below. Asillustrated in FIG. 2, the HMD may include two separate FS/UWA/ROsurfaces 120R and 120L for the user's two eyes which are separatelysupported by the frame and/or a nasal ridge piece 210 (see below).Alternately, the HMD may employ a single FS/UWA/RO surface that servesboth eyes with a single structure, some portions of which are viewed byboth eyes and other portions of which are viewed by only one eye.

As noted immediately above and as illustrated in FIG. 2, thehead-mounted display apparatus 100 can include a nasal ridge piece 210.The nasal ridge piece can be a vertical bar or wall which provides aseparation between two FS/UWA/RO surfaces, one for each of the user'seye. The nasal ridge piece 210 can also provide a separation between thefields of view of the user's two eyes. In this way, the user's right eyecan be shown a first representation of three dimensional physicalreality in the environment by displaying a first image to the right eyevia a first image display device and a first FS/UWA/RO surface, whilethe user's left eye is shown a second representation of threedimensional physical reality in the environment by displaying a secondimage to the left eye via a second image display device and a secondFS/UWA/RO surface. A separate display device/reflective surfacecombination thus services each eye of the user, with each eye seeing thecorrect image for its location relative to the three dimensionalphysical reality in the environment. By separating the user's two eyes,the ridge piece 210 allows the image applied to each eye to be optimizedindependently of the other eye. In one embodiment, the nasal ridgepiece's vertical wall may include two reflectors, one on each side, toallow the user to see imagery as he/she turns his/her eyes nasally,either to the left or to the right.

The at least one image display system 110 can be mounted inside theFS/UWA/RO surface 120 and may be horizontally disposed or at a slightangle with respect to the horizon. Alternatively, the at least one imagedisplay system can be located just outside of the FS/UWA/RO surface. Thetilt or angle of the at least one image display system 110 or, moreparticularly, its at least one light-emitting surface, will in generalbe a function of the location of the pixels, images, and/or pieces ofdisplay information that are to be reflected from the surface 120.

In certain embodiments, the head-mounded display apparatus 100 isconfigured to create an interior cavity, with the FS/UWA/RO surfacebeing reflective inward into the cavity. For a FS/UWA/RO surface havingtransmissive properties, the image or display information from the atleast one image display system is reflected into the cavity and to theuser's eye from the surface while, simultaneously, light also enters thecavity and the user's eye from the external world by passing through thereflective surface.

The head-mounted display apparatus can include an electronics package140 to control the images that are displayed by the at least one imagedisplay system 110. In one embodiment, the electronics package 140includes accelerometers and gyroscopes that provide location,orientation and position information needed to synchronize images fromthe at least one image display system 110 with user activities. Powerand video to and from the head-mounted display apparatus 100 can beprovided through a transmission cable 150 coupled to the electronicspackage 140 or through a wireless medium.

A set of cameras 170 may be situated on opposite sides of thehead-mounted display apparatus 100 to provide input to the electronicspackage to help control the computer generation of, for example,“augmented reality” scenes. The set of cameras 170 may be coupled to theelectronics package 140 to receive power and control signals and toprovide video input to the electronics package's software.

The image display system used in the head-mounted display apparatus cantake many forms, now known or subsequently developed. For example, thesystem can employ small high resolution liquid crystal displays (LCDs),light emitting diode (LED) displays, and/or organic light emitting diode(OLED) displays, including flexible OLED screens. In particular, theimage display system can employ a high-definition small-form-factordisplay device with high pixel density, examples of which may be foundin the cell phone industry. A fiber-optic bundle can also be used in theimage display system. In various embodiments, the image display systemcan be thought of as functioning as a small screen television. If theimage display system produces polarized light (e.g., in the case wherethe image display system employs a liquid crystal display where allcolors are linearly polarized in the same direction), and if theFS/UWA/RO surface is polarized orthogonally to the light emitted by thedisplay, then light will not leak out of the FS/UWA/RO surface. Theinformation displayed and the light source itself will accordingly notbe visible outside of the HMD.

Overall operation of an exemplary embodiment of an optical systemconstructed in accordance with the present disclosure, specifically, anoptical system for an “augmented reality” HMD, is illustrated by theray-tracings of FIG. 1, specifically, light rays 180, 185, and 190. Inthis embodiment, FS/UWA/RO surface 120 has both reflective andtransmissive properties. Using surface 120's transmissive properties,light ray 190 enters from the environment through the surface andproceeds towards the user's eye. From the same region of surface 120,light ray 180 is reflected by the surface (using the surface'sreflective properties) and joins light ray 190 to create combined lightray 185 that enters the user's eye when the user looks in the directionof point 195, i.e., when the user's direction of gaze is in thedirection of point 195. While so looking, the user's peripheral visioncapabilities allow the user to see light from other points in theenvironment which pass through surface 120, again using the surface'stransmissive properties.

III. Fresnel Lens Systems

In accordance with the present disclosure, the images and/or pieces ofdisplay information provided by the at least one image display systemare adjusted for near viewing prior to entering the user's eye(s). Forexample, in the exemplary embodiment of FIGS. 1 and 2, the adjustment isperformed by lens system 115 which includes one or more Fresnel lenselements and serves to modify the diopter characteristics of the lightbeam emanating from the display surface thus making it easier for theuser to focus on the virtual image of the display produced by theoverall optical system. FIGS. 12-14 and 15-18 show other embodimentsemploying Fresnel lens elements to modify the diopter characteristics oflight emanating from a display. In addition to this function, theFresnel lens elements also serve to magnify the image provided to theuser. In some embodiments, a magnification of between three to six ormore may be obtained with multiple Fresnel lens elements arranged in astacked configuration.

As discussed in more detail below, in certain embodiments, the Fresnellens system contains one or more curved Fresnel lens elements, i.e.,Fresnel lenses constructed on curved, rather than flat, substrates. Forease of reference, Fresnel lens systems that include a curved Fresnellens element will be referred to herein as “curved Fresnel lenssystems,” it being understood that not all of the Fresnel lens elementsused in a curved Fresnel lens system need be curved. The phrase “Fresnellens system” will be used to describe the general case of a lens systemthat includes at least one Fresnel lens element (whether curved or flat)which performs the function of modifying the diopter characteristics ofthe light beam emanating from an image display system to facilitatenear-to-the-eye viewing of an image of the display. As discussed in moredetail below, in embodiments that employ a FS/UWA/RO surface, ifdesired, the FS/UWA/RO surface can also have optical properties thatcontribute to in-focus, near-to-the-eye viewing of images formed on theat least one light-emitting surface of the image display system.

In general terms, the Fresnel lens systems disclosed herein can comprisevarious combinations of flat and/or curved Fresnel lenses selected toadjust the diopter of the light emanating from the image display systemso as to allow the eye to be able to focus on the display and, in thecase of “augmented reality” HMDs, also focus on objects in the externalenvironment. The presence of at least one curved Fresnel lens in acurved Fresnel lens system provides at least one additional parameter(i.e., the curvature of the lens) for controlling aberrations in theimage provided to the user. For example, one or more Fresnel lenseshaving curved configurations can provide substantial reductions inchromatic aberrations. Furthermore, Fresnel surfaces manufactured oncurved substrates can provide reduced off-axis aberrations.

More generally, the optical properties of the Fresnel lens system andthe one or more Fresnel lenses included therein can be selectedempirically or through analytic ray-tracing. Ray-tracing can, forexample, allow optimization of the apparatus parameters for a particularimplementation, such as military training, flight simulation, gaming andother commercial applications. The parameters that are available foroptimization include, for example, the curvature of the display, thedimensions of the display, the curvature of the Fresnel lenses, asphericparameters in cases where the Fresnel lens system or other parts of theoptical system include one or more aspheric surfaces, and the Fresnellens power versus the distances from (i) the front of the display screenand (ii) the user's eye.

In some embodiments, the Fresnel lens elements produce no fieldcurvature, so a wide field of view can be provided using a small numberof thin optical components. In other embodiments, the Fresnel lenssystem can include one or more aspheric surfaces to aid in thecorrection of image aberrations. The aspheric surface can be applied oneither surface of any of the optical components of the Fresnel lenssystem. Nominally, the first and second surfaces of the Fresnel lenselements will have the same base radius of curvature (i.e., theirthickness will be constant over their clear aperture). Additionalaberration correction or functionality may be achievable by allowing oneor more of the Fresnel lens elements to have different radii on theirfirst and second surfaces.

In various embodiments, through the use of Fresnel lens elements,including aspheric Fresnel lens elements, an optical system can berealized as a compact and lightweight system, having a large viewablefield of view, an image quality commensurate with typical human visualresolution, and an overall structure that can be manufactured in largequantities at low cost. If desired, the Fresnel lens system can includeone or more diffractive surfaces (diffractive components) to reducechromatic aberrations, particularly, lateral chromatic aberration. Forexample, lens elements 810, 1330, and 1135 may include one or morediffractive surfaces. In this way, a corrected image of an image displaydevice, including a flat image display device, can be achieved eitherusing the Fresnel lens system alone or in combination with a FS/UWA/ROsurface. In certain embodiments, the one or more Fresnel lenses willprovide most of the optical power in the system and will be designed tominimize monochromatic aberrations.

The Fresnel lens elements, which in one embodiment are assembled with agap between adjacent lenses, may be made much thinner than typicallenses, so the space and weight of the optical system is significantlyreduced compared to conventional thick lenses. The weight may be furtherreduced by making all lenses from plastic. However, glass could also beused. The plastic lenses can be produced by either diamond machining ormolding.

In certain embodiments, one or more (or all) of the curved Fresnel lenselements can have facets whose edges lie along radial lines that passthrough the center of rotation of a nominal user's eye. FIG. 3illustrates such an embodiment where 30 is the Fresnel lens, 31 is afacet of the Fresnel lens, 32 is an edge of a facet of the Fresnel lens,and 33 is a radial line which passes through the center of rotation 34of a nominal user's eye 35. FIG. 3 also shows the internal lens 36(natural lens 36) of the nominal user's eye. Alternatively, one or more(or all) of the curved Fresnel lens elements can have facets whose edgeslie along radial lines that pass through the center of a nominal user'snatural lens or are normal to the surface of a nominal user's cornea.

As noted above, Fresnel lenses are particularly well-suited for use inHMDs because of their light weight. The lenses can, however, createimage aberrations due to the angle of incidence of the light wavesleaving the display on the lens surface. In particular, the light wavescan pass through unintended sections of the Fresnel lens grooves. Inaccordance with the embodiment illustrated in FIG. 3, such aberrationscan be reduced by providing the Fresnel lens with a domed shape,specifically, a spherical shape, centered on the center of rotation of anominal user's eye, such that the edges of the Fresnel facets are normalto the surface of the dome everywhere around the lens' surface.Alternatively, the domed shape (spherical shape) can be centered on thecenter of a nominal user's natural lens or can be concentric with anominal user's cornea. In these ways, light beams pass through the lensparallel to the edges of the facets and optical aberrations due to thesediscontinuities are avoided, improving, among other things, the colorresponse of the lens. It should be noted that convergent facet edgeswill reduce optical distortion in a viewed image even if all of theedges do not exactly satisfy one of the above conditions, e.g., if allof the edges do not exactly pass through the center of rotation of anominal user's eye. Accordingly, rather than having a pure sphericalshape, the Fresnel lens can be substantially spherical (e.g., theFresnel lens can have an aspheric surface) and can still benefit fromhaving at least some convergent facet edges.

Although Fresnel lens elements having square, rectangular, or otherclear aperture shapes can be used if desired, in general, the Fresnellenses will have circular clear apertures. In most applications, thesize of the smallest clear aperture of the lenses making up the Fresnellens system will determine whether or not the overall optical system isa pupil forming or a non-pupil forming system. In particular, for anoverall optical system composed of a Fresnel lens system and a FS/UWA/ROsurface, the exit pupil of the system will typically be the image of thesmallest clear aperture of the Fresnel lens system produced by theoptical elements downstream of that aperture (i.e., towards the user'seye). That is, the system's overall aperture stop will typically be inthe Fresnel lens system because in terms of apertures, FS/UWA/ROsurfaces behave as if they have very large clear apertures. Depending onthe size and location of the image of the smallest clear aperture of theFresnel lens system produced by the FS/UWA/RO surface (as well as by anyelements of the Fresnel lens system on the downstream side of theelement with the smallest clear aperture), the overall system mayprovide the user with a full foveal dynamic field of view, a fullfoveal+peripheral static field of view, or a full foveal+peripheraldynamic field of view.

FIG. 4 shows an embodiment of a HMD optical system which employs aFS/UWA/RO surface and a Fresnel lens system 115 having a flat Fresnellens 810 and two curved Fresnel lenses 815 and 820, which, as shown inFIG. 4, are adjacent to one another. Light rays 830, 835, and 840 areshown in this figure with light 840 entering from the environment andbecoming combined with light 830 to create combined light 835 thatenters the user's eye when the user looks in the direction of point 850.The user's peripheral vision capabilities also allow the user to seelight from points other than point 850.

More particularly, a diverging wavefront of light 860 emanating from theat least one image display system 110 is converged in a positive-diopterFresnel lens system having Fresnel lenses 810, 815, and 820 to providelight 830 that is between zero diopter and the initial diopter. Theinitial diopter of the light emanating from the at least one imagedisplay system 110 can, for example, be approximately D=1/(0.03 [m])=33dpt. After leaving the Fresnel lens system, the light reflects fromFS/UWA/RO surface 120 where, if desired, additional diopter divergencecan be removed using the surface curvature techniques discussed below.

The total diopter change can, for example, be 33 dpt, and this may besplit between the FS/UWA/RO surface and the Fresnel lenses in variousembodiments. In particular, the amount of diopter change supplied by theFS/UWA/RO surface can be reduced which in various embodiments may beadvantageous in designing and manufacturing the FS/UWA/RO surface.Because diopters are additive, however much vergence is supplied by oneof the optical components does not have to be supplied by the other.(This additive property of diopter values can be used in combining thecollimating effects of the Fresnel lens system and the FS/UWA/ROsurface, as well as in combining the effects of the individual lenselements making up the Fresnel lens system. It can also be used to takeaccount of the collimating effects of any other optical components thatmay be part of the overall system.) In the exemplary embodiment of FIG.4, a diopter change of 33 dpt will result in a final beam that iscollimated (0 dpt) or substantially collimated (˜0) dpt). This isequivalent to light coming from a point essentially infinitely distant,and the light wavefront will be flat, resulting in parallel surfacenormals to the wavefront, shown as rays 835, across the entrance to theeye. Collimated reflected light can, for example, be desirable when theexternal world includes items that are effectively at infinity relativeto the user. As noted above, the FS/UWA/RO surface 120 admits light ray840 from the external environment, thus allowing the internal images tooverlay the external images and, in particular, external images whichare effectively at infinity relative to the user's eye.

As discussed above, prior optical systems used in HMDs that haveemployed reflective optical surfaces have been pupil forming and thushave had limited viewing areas, a typical field of view being ˜60degrees or less. This has greatly limited the value and capability ofprior head-mounted display apparatuses. In various embodiments, thehead-mounted displays disclosed herein have much wider fields of view(FOV), thus allowing much more optical information to be provided to theuser compared to HMDs having smaller fields of view. The wide field ofview can be greater than 100°, greater than 150°, or greater than 200°.In addition to providing more information, the wide field of view allowsthe additional information may be processed by the user in a morenatural manner, enabling better immersive and augmented realityexperiences through a better match of the displayed images to physicalreality.

Specifically, in the exemplary embodiment illustrated in FIG. 5, for astraight ahead direction of gaze, the eye is able to take in a wholeviewing area represented in FIG. 5 by curved FS/UWA/RO surfaces 201 and202, corresponding to at least 150 degrees of horizontal field of view(FOV) for each eye (e.g., ˜168 degrees of horizontal FOV). This field ofview is composed of the eye's foveal field of view and its peripheralfield of view. In addition, the eye is allowed to move freely about itscenter of rotation to aim the combined foveal+peripheral field of viewin different directions of gaze, as the eye naturally does when viewingthe physical world. The optical systems disclosed herein thus allow theeye to obtain information throughout a range of motion in the samemanner as the eye does when viewing the natural world.

Examining FIG. 5 in more detail, this figure is a simplified linerepresentation of the front of a user's head 200 as seen from the top.It shows FS/UWA/RO surfaces 201 and 202 placed in front of the user'seyes 203 and 204. As discussed above, the FS/UWA/RO surfaces 201 and 202may rest upon the user's nose 205 where they come together at the centerfront 214 of the user's head 200. As discussed in detail below, thelocal normals and local spatial locations of surfaces 201 and 202 areadjusted so that images produced by the at least one image displaysystem (not shown in FIG. 5) cover at least 100°, e.g., in certainembodiments, at least 150° and, in other embodiments, at least 200°, ofhorizontal FOV for each eye. (Optionally, as also discussed below, thelocal radii of curvature are also adjusted to provide, when combinedwith a Fresnel lens system, distant virtual images.) For example, thelocal normals and local spatial locations can be adjusted to cover theuser's complete ˜168 degree, straight ahead, horizontal, static field ofview for each eye, with the 168 degrees extending from edge-to-edge ofthe FS/UWA/RO surfaces 201 or 202, as shown by sight lines 210,211 and212,213. The sight lines thus correspond to the wide static field ofview (foveal+peripheral) that is provided to the user. In addition, theuser is free to move his/her eyes around rolling centers 215 and 216while continuing to see computer-generated imagery.

In FIG. 5, as well as in FIG. 11, the FS/UWA/RO surfaces are shown asparts of spheres for ease of presentation. In practice, the surfaces arenot spheres, but have more complex configurations so that their localnormals and local spatial locations (and, optionally, local radii ofcurvature) will provide the desired static and dynamic fields of view(and, optionally, desired distances to the virtual images). Also, inFIG. 5, the right side of the head-mounted display apparatus operatesidentically to left side, it being understood that the two sides candiffer if desired for particular applications. Also for ease ofpresentation, FIGS. 5-11 do not show an optical system which includes atleast one Fresnel lens between the at least one image display system andthe reflective optical surface, it being understood that in accordancewith the present disclosure, such an optical system is used in theembodiments disclosed herein.

FIGS. 6 and 7 further illustrate the static and dynamic fields of viewprovided by the FS/UWA/RO surfaces disclosed herein. FIG. 6 shows auser's nominal right eye 71 having a straight ahead direction of gaze73. The eye's foveal+peripheral field of view is shown by arc 75, whichhas an angular extent of ˜168°. Note that for ease of presentation, inFIGS. 6-8, the field of view is shown relative to the center of rotationof the user's eye as opposed to the center or edges of the user's pupil.In fact, the large field of view (e.g., ˜168°) achieved by a human eyeis a result of the large angular extent of the retina which allowshighly oblique rays to enter the user's pupil and reach the retina.

FIG. 7 schematically shows the interaction of the field of view of FIG.6 with a HMD having: (a) an image display system whose at least onelight-emitting surface 81 has a first light-emitting region 82(illustrated as a square) and a second light-emitting region 83(illustrated as a triangle) and (b) a FS/UWA/RO surface having a firstreflective region 84 which has a first local normal 85 and a secondreflective region 86 which has a second local normal 87.

As indicated above, the FS/UWA/RO surface is both a “free space” surfaceand an “ultra-wide angle” surface. In addition, as noted above anddiscussed in more detail below, the surface can participate incollimation (or partial collimation) of the light that enters the user'seye. Such collimation causes the virtual image produced by the FS/UWA/ROsurface and the Fresnel lens system to appear to be located a longdistance from the user, e.g., 30 meters or more, which permits the userto easily focus on the virtual image with a relaxed eye.

The “free space” and “ultra-wide angle” aspects of the FS/UWA/RO surfacecan be achieved by adjusting the local normals of the surface so thatthe user's eye sees light-emitting regions of the at least one imagedisplay system as coming from predetermined regions of the FS/UWA/ROsurface (predetermined locations on the surface).

For example, in FIG. 7, the designer of the HMD might decide that itwould be advantageous for a virtual image 88 of the square to be viewedby the center portion of the user's retina when the user's direction ofgaze is straight ahead and for a virtual image 89 of the triangle to beviewed by the center portion of the user's retina when the direction ofgaze is, for example, ˜50° to the left of straight ahead. The designerwould then configure the at least one image display system, theFS/UWA/RO surface, the Fresnel lens system and any other opticalcomponents of the system so that the virtual image of the square wouldbe straight ahead and the virtual image of the triangle would be 50° tothe left of straight ahead during use of the HMD.

In this way, when the user's direction of gaze (line of sight)intersected the FS/UWA/RO surface straight on, the virtual image of thesquare would be visible at the center of the user's eye as desired, andwhen the user's direction of gaze (line of sight) intersected theFS/UWA/RO surface at 50 degrees to the left of straight ahead, thevirtual image of the triangle would be visible at the center of theuser's eye, as also desired. Although not illustrated in FIGS. 6 and 7,the same approaches are used for the vertical field of view, as well asfor off-axis fields of view. More generally, in designing the HMD andeach of its optical components, the designer “maps” the at least onelight-emitting surface of the display to the reflective surface so thatdesired portions of the display are visible to the user's eye when theeye's gaze is in particular directions. Thus, as the eye scans acrossthe field of view, both horizontally and vertically, the FS/UWA/ROsurface shines different portions of the at least one light emittingsurface of the image display system into the user's eye. Although theforegoing discussion has been in terms of the center of a nominal user'sretina, the design process can, of course, use the location of a nominaluser's fovea instead, if desired.

It should be noted that in FIG. 7, any rotation of the user's eye toright causes the virtual image 89 of the triangle to no longer bevisible to the user. Thus, in FIG. 7, any direction of gaze that isstraight ahead or to the left of straight ahead provides the user withvirtual images of both the square and the triangle, while a direction ofgaze to the right of straight ahead provides a virtual image of only thesquare. The acuity of the virtual images will, of course, depend onwhether the virtual images are perceived by the user's foveal vision orthe user's peripheral vision.

If the designer of the HMD had placed the virtual image of the squarefar to the right in FIG. 7 while leaving the virtual image of thetriangle far to the left, there would be directions of gaze where onlythe virtual image of the square was visible and other directions of gazewhere only the virtual image of the triangle was visible. Likewise,based on the principles disclosed herein, the designer could arrange thevirtual image of the square and the virtual image of the triangle sothat the virtual image of the triangle was always visible, with thevirtual image of the square being visible for some directions of gaze,but not for others. As a further variation, the designer of the HMDcould place the virtual image of the square and triangle at locationswhere for one or more directions of gaze, neither image was visible tothe user, e.g., the designer could place the virtual images just outsidethe user's static field of view for a straight ahead direction of gaze.The flexibility provided to the HMD designer by the present disclosureis thus readily apparent.

In one embodiment, the “free space” and the “ultra-wide angle” aspectsof the reflective surface are achieved by using the principles of Fermatand Hero pursuant to which light travels along the shortest (least time)optical path. Commonly-assigned and co-pending U.S. patent applicationSer. No. 13/211,389, filed simultaneously herewith in the names of G.Harrison, D. Smith, and G. Wiese, entitled “Methods and Systems forCreating Free Space Reflective Optical Surfaces,” the contents of whichare incorporated herein by reference, describes an embodiment in whichthe Fermat and Hero principles are used to design FS/UWA/RO surfacessuitable for use in HMDs. See also commonly-assigned and co-pending U.S.patent application Ser. No. 13/211,372, filed simultaneously herewith inthe names of G. Harrison, D. Smith, and G. Wiese, entitled “Head-MountedDisplay Apparatus Employing One or More Reflective Optical Surfaces,”the contents of which are also incorporated herein by reference.

By means of the Fermat and Hero least-time principles, any “desiredportion” of the at least one light-emitting surface of an image displaysystem (e.g., any pixel of an image display system) can be caused tohave any desired point of reflection at the FS/UWA/RO surface, providedthat the optical path from the desired portion of the at least onelight-emitting surface to the point of reflection at the FS/UWA/ROsurface and then to the center of rotation of the user's eye is at anextremum.

An extremum in the optical path means that the first derivative of theoptical path length has reached a zero value, signifying a maximum or aminimum in the optical path length. An extremum can be inserted at anypoint in the field of view by creating a local region of the reflectiveoptical surface whose normal bisects (a) a vector from the local regionto the user's eye (e.g., a vector from the center of the local region tothe center of the user's eye) and (b) a vector from the local region tothe “desired portion” of the light-emitting surface (e.g., a vector fromthe center of the local region to the center of the “desired portion” ofthe light-emitting surface). FIGS. 8 and 9 illustrate the process forthe case where the “desired portion” of the at least one light-emittingsurface of the image display system is a pixel.

Specifically, FIG. 8 shows a light-emitting surface 510 of an imagedisplay system composed of a generally rectangular array of pixels thatare emanating light toward the front of a head-mounted display apparatusin the direction of light beam 515. Light beam 515 bounces off ofreflective optical surface 520, which for ease of presentation is shownas a flat in FIG. 8. Upon reflection, light beam 515 becomes light beam525 that enters the user's eye 530.

For the purposes of determining the surface normal of the reflector foreach pixel, it is only necessary to determine the three-dimensionalbisector of vectors corresponding to light beams 515 and 525. In FIG. 8,this bisector vector is shown in two-dimensional form as line 535.Bisecting vector 535 is normal to the reflective optical surface atpoint of reflection 540, which is the location on surface 520 wherepixel 545 of light-emitting surface 510 will be visible to the user ofthe HMD.

Specifically, in operation, pixel 545 in the display surface 510 emitslight beam 515 that bounces off reflective optical surface 520 at anangle established by the surface normal corresponding to bisectingvector 535 and its perpendicular plane 550, yielding by the Fermat andHero principles, a reflected pixel at point of reflection 540 that isseen by the eye 530 along light beam 525. In order to accuratelycalculate the surface normal at the point of reflection 540, the beam525 can pass through approximately the center 555 of the user's eye 530.The results will remain approximately stable even if the user's eyerotates, becoming peripheral vision until, as discussed above inconnection with FIGS. 6 and 7, the eye turns so much that that region ofthe display cannot be seen with either the user's foveal or peripheralvision.

To calculate the position of the surface normal, the use of the methodof quaternions may be employed, where

-   -   q1=orientation of beam 515    -   q2=orientation of beam 525        and    -   q3=the orientation of the desired surface normal 535=(q1+q2)/2

The surface normal may also be described in vector notation, asillustrated in FIG. 10. In the following equation and in FIG. 10, pointN is one unit away from the point M at the center of the region ofinterest of the reflective optical surface and is in the direction ofthe perpendicular normal to the tangent plane of the reflective opticalsurface at the point M. The tangent plane of the reflective opticalsurface at point M is controlled to satisfy the relation expressed inthe following equation such that in three-dimensional space, the surfacenormal at the point M bisects the line from the point M to the point Pat the center of the pixel of interest and the line from point M to thepoint C at the rolling center of the user's eye (for reference, point Cis approximately 13 mm back from the front of the eye).

The equation describing the point N on the surface normal at point M is:

$N = {\frac{\left( {P - M} \right) + \left( {C - M} \right)}{{\left( {P - M} \right) + \left( {C - M} \right)}} + M}$where all the points, N, M, P, and C have components [x, y, z] thatindicate their position in three-dimensional space in an arbitraryCartesian coordinate system.

The resulting normal vector N-M has the Euclidean length|N−M|=1where the two vertical bars represents the Euclidean length, calculatedas follows:|N−N|=√{square root over ((x _(N) −x _(M))²+(y _(N) −y _(M))²+(z _(N) −z_(M))²)}.

As a numerical example, consider the following M, P, and C values:

-   -   M=[x_(M),y_(M),z_(M)]=[4,8,10]    -   P=[2,10,5]    -   C=[6,10,5]        The point along the normal, N, is calculated as follows:

P − M = [(2-4), (10-8), (5-10)] = [−2, 2, −5]C − M = [(6-4), (10-8), (5-10)] = [2, 2, −5](P − M) + (C − M) = [0, 4, −10] and $\begin{matrix}{N = {\frac{\left( {P - M} \right) + \left( {C - M} \right)}{{\left( {P - M} \right) + \left( {C - M} \right)}} + M}} \\{= {{\left\{ {\left\lbrack {{- 2},2,{- 5}} \right\rbrack + \left\lbrack {2,2,{- 5}} \right\rbrack} \right\}/10.7703296143} + \left\lbrack {4,8,10} \right\rbrack}} \\{= {\left\lbrack {0,0.3713806764\;,{- 0.928476691}}\; \right\rbrack + \left\lbrack {4,8,10} \right\rbrack}} \\{= \left\lbrack {4,8.3713806764\;,9.0715233091}\; \right\rbrack}\end{matrix}$The geometry is shown in FIG. 19, where the bisector is between the twolonger vectors.

The foregoing is, of course, merely a representative calculation servingto show the use of the Fermat and Hero principles of least time indetermining local tangent plane angular constraints for a field ofpoints making up a free-space (free-form) surface manifold of reflectingregions intended to present a contiguous virtual image to the viewer.The only real constant is the center of the user's eye, and the eye'snatural field of view. All other components may be iteratively updateduntil an appropriate solution for a given image display system andreflective optical surface orientation is reached. Looked at anotherway, the pixel image reflection locations, M1, M2, . . . , Mn, and theirassociated normals and curvatures may be thought of as a matrix that is“warped” (adjusted) so that the FS/UWA/RO surface achieves the desiredvirtual image processing of computer-generated images formed by theimage display system.

In applying the Fermat and Hero principles, it should be noted that insome embodiments, it will be desirable to avoid the situation where thenormals are adjusted such that the user sees the same pixel reflectionat more than one point. It should also be noted that in someembodiments, the local regions of the reflective optical surface can bevery small and may even correspond to a point on the reflector, with thepoints morphing into other points to make a smooth surface.

To facilitate the presentation, the effects of the presence of a Fresnellens system has not been explicitly included in the above discussion ofthe use of the Fermat and Hero principles to design a FS/UWA/RO surface.In practice, the presence of a Fresnel lens system is readily includedin the analysis by using as the input to the Fermat and Herocalculations, the directions of propagation of the light beams afterthey have passed through the optical elements making up the Fresnel lenssystem (or any other optical elements used in the overall opticalsystem). Those directions of propagation can, for example, be determinedusing Gaussian optics techniques. If desired, the Fermat and Herocalculations can be repeated for different initial light vergencesettings as controlled by changing the Fresnel lensing power of theFresnel lens system until desired virtual images are obtained.

In order to ensure that the user can easily focus on the virtual imageof the “desired portion” of the at least one light-emitting surface(e.g., the virtual image of a pixel), in certain embodiments, the radiusof curvature of the region surrounding the reflection point (reflectionarea) is controlled so that after passing through the Fresnel lenssystem and reflecting from the FS/UWA/RO surface, a collimated (or nearcollimated) image reaches the user. As noted above, a collimated (ornear collimated) image has optical rays that are more parallel, as ifthe image had originated at a far distance from the user, tens tohundreds of meters for instance. In order to achieve such a surface,depending on the collimating power of the Fresnel lens system, theradius of curvature of the reflection region of the reflective opticalsurface corresponding to the “desired portion” of the at least onelight-emitting surface (desired light-emitting pixel) may be kept to aradius on the order of (but greater than) one-half the distance from thereflection region to the actual “desired portion” of the light-emittingsurface (actual pixel) on the display. More particularly, the radiuswill be on the order of one-half the apparent distance from thereflection region to the “desired portion” of the light-emitting surfacewhen the “desired portion” is viewed through the Fresnel lens systemfrom the location of the reflection region.

Thus, in one embodiment, the inter-reflected-pixel normal vector fromthe pixel of concern to the adjacent pixels satisfies a relationshipthat allows them to establish a radius of curvature on the order ofapproximately one-half the length of the vector from the location of thereflected pixel on the reflective surface to the apparent location ofthe display pixel as seen through the Fresnel lens system. Adjustmentsthat affect this parameter include the size of the at least one lightemitting surface and whether the at least one light emitting surface iscurved.

FIG. 9 illustrates this embodiment. In order to control the radius ofcurvature of the region surrounding the pixel reflection so that acollimated (or near collimated) image reaches the user, two adjacentpixel reflecting regions, such as at the point of reflection 540, areconsidered. More regions may be considered for better balance but twoare sufficient. With reference to FIG. 9, two pixel reflective points540 and 610 are shown with respect to two pixels, 545 and 615,respectively on display surface 510. The surface normals at points 540and 610 are calculated along with the angle between their directions.The radius of curvature is calculated knowing these angles and thedistance between the points 540 and 610. Specifically, the surfaceconfiguration and, if needed, the surface's spatial location areadjusted until the radius of curvature is on the order of approximatelyone-half of the average of the lengths of beams 515 and 620 when thoselengths are adjusted for the effects of the Fresnel lens system. In thisway, zero or near-zero diopter light can be provided to the user's eye.As noted above, this is equivalent to light coming from a pointessentially infinitely distant, and the light wavefront is flat,resulting in parallel surface normals to the light's wavefront.

In addition to controlling the local radii of curvature, in certainembodiments, as a first order point solution to having a collimated (ornear collimated) image enter the eye, the at least one light emittingsurface is nominally located a distance of one focal length away fromthe FS/UWA/RO surface, where the focal length is based on the averagevalue of the radii of curvature of the various reflective regions makingup the FS/UWA/RO surface.

The result of applying the Fermat and Hero principles is a set ofreflective regions that may be combined into a smooth reflectivesurface. This surface will, in general, not be spherical or symmetric.FIG. 11 is a two dimensional representation of such a FS/UWA/RO surface520. As discussed above, surface 520 may be constructed such that theradii of curvature at points 710 and 720 are set to values which, whencombined with the collimating effects of the Fresnel lens system,provide for relaxed viewing of the image from the at least onelight-emitting surface of the image display system which is beingreflected by the surface. In this way, looking in a certain directionrepresented by line 730 will provide a collimated (or near collimated)virtual image to the eye 530, as will looking in a different directionrepresented by line 740. To enable a smooth transition of viewing allacross the field of view, the regions of the FS/UWA/RO surface may besmoothly transitioned from one control point to another, as may beperformed by using Non-Uniform Rational B-Spline (NURBS) technology forsplined surfaces, thus creating a smooth transition across thereflective surface. In some cases, the FS/UWA/RO surface may include asufficient number of regions so that the surface becomes smooth at afine grain level. In some embodiments, different magnifications for eachportion of the display (e.g., each pixel) may be provided using agradual gradient to allow better manufacturability, realization, andimage quality.

From the foregoing, it can be seen that the overall head-mounted displaycan be designed using the following exemplary steps: determining adesired field of view, choosing a display surface size (e.g., width andheight dimensions), choosing an orientation for the display surfacerelative to a reflective surface, choosing a candidate location for theFresnel lens system between the display and the reflective surface,choosing a candidate configuration for a Fresnel lens system, catalogingthe position of every pixel on the display surface as seen through theFresnel lens system, and choosing a location for display of every pixelfrom the display surface on the reflective surface. The display surfaceand the Fresnel lens system can be placed above the eye and tiltedtoward the reflective surface, allowing the curvature of the reflectivesurface to reflect light to the eye of the wearer. In furtherembodiments, the display surface and Fresnel lens system may be placedin other positions, such as to the side of the eye or below the eye,with the reflective position and curvature selected to reflect the lightfrom the display surface appropriately, or being tilted to a differentdegree.

In certain embodiments, a three-dimensional instantiation ormathematical representation of the reflective surface can be created,with, as discussed above, each region of the reflective surface being alocal region having a normal that bisects the vectors from the center ofthat region to the center of the user's eye and to the center of a pixelin the display surface (center of the apparent position of the pixelresulting from the presence of the Fresnel lens system). As alsodiscussed above, the radii of curvature of regions surrounding a pixelreflection can be controlled so that in combination with the collimatingeffects of the Fresnel lens system, a collimated (or near collimated)image reaches the user across the field of view. Through computer-basediterations, changeable parameters (e.g., local normals, localcurvatures, and local spatial locations of the reflective surface andthe number of elements, the powers of the elements, the curvatures ofthe elements, and the locations of elements for the Fresnel lens system)can be adjusted until a combination (set) of parameters is identifiedthat provides a desired level of optical performance over the field ofview, as well as a manufacturable design which is aestheticallyacceptable.

During use, a non-symmetrical FS/UWA/RO surface (which, in certainembodiments, is constructed from a splined surface of multiple localregions of focus) in combination with a Fresnel lens system forms avirtual image of the at least one light emitting surface of the imagedisplay system that is stretched across a wide field of view. TheFS/UWA/RO surface may be thought of as a progressive mirror orprogressive curved beam splitter or a free-form mirror or reflector. Asthe eye scans across the field of view, both horizontally andvertically, the curved FS/UWA/RO surface shines different portions ofthe at least one light-emitting surface of the image display system intothe user's eye. In various embodiments, the overall optical system ismanufacturable in large quantities at low cost while maintaining animage quality commensurate with typical human visual resolution.

IV. HMDs that Employ Non-FS/UWA/RO Reflective Surfaces

As noted above, FIG. 4 shows an embodiment of a HMD optical system whichuses a curved FS/UWA/RO surface and a curved Fresnel lens system. HMDoptical systems that employ curved reflective surfaces that are notFS/UWA/RO surfaces, as well as those employing flat reflective surfaces,can also benefit from the use a curved Fresnel lens system between animage display system and the reflective surface. FIGS. 12-14 show anexemplary embodiment which employs a flat reflective surface and acurved Fresnel lens system.

In FIG. 12, a user 1300 is shown wearing a head-mounted display whichincludes an optical see-through, augmented reality binocular viewer1310. Viewer 1310 includes at least one image display system 1320, atleast one reflective surface 1380, and at least one curved Fresnel lenssystem that provides near viewing of the display and a wide field ofview. Typically, viewer 1310 will include one display system/curvedFresnel lens system/reflective surface combination for each eye,although one or more of these components can service both eyes ifdesired.

As shown in FIG. 12, the curved Fresnel lens system includes Fresnellenses 1330 and 1335. Both a flat Fresnel lens 1330 and curved Fresnellens 1335 may be employed in various embodiments to provide a field ofview of 100 degrees or more. As with the other exemplary embodimentsdiscussed herein, more or fewer lenses than shown in FIG. 12 may be usedin the curved Fresnel lens system. In one embodiment, a single curvedFresnel lens element can be used. Note that in embodiments that employ aFS/UWA/RO surface, a single Fresnel lens element, e.g., a single curvedFresnel lens element, can be used. In another embodiment, illustrated inFIGS. 13 and 14, three Fresnel lens elements 1125, 1130, and 1135 areused.

An electronics package 1340 is provided for controlling the image thatis displayed by the at least one image display system 1320. Theelectronics package 1340 may include accelerometers and gyroscopes forlocating and positioning the user. Power and video to and from thebinocular viewer can be provided through a transmission cable 1350 orwireless medium. A set of cameras 1370 may be situated on opposite sidesof the user's head to provide input to the HMD's software package tohelp control the computer generation of augmented reality scenes.

The optical see-through, augmented reality binocular viewer 1310includes at least one reflective optical surface 1380 that allows atleast one internally-generated image to overlay at least one imageentering the viewer from the external environment. In particular, light1386 enters the viewer from the external environment by passing throughreflective optical surface 1380. This light combines with light 1385from the image display system and the curved Fresnel lens system whichhas been reflected by reflective optical surface 1380 towards the user'seye. The result is combined light 1387 that enters the user's eye whenthe user looks in the direction of point 1390. The user's peripheralvision capabilities allow the user to see light from other parts ofreflective optical surface 1380, distant from point 1390.

In one embodiment, as shown, the at least one image display system 1320and the curved Fresnel lens system (e.g., Fresnel lenses 1330 and 1335)are supported above the user's eye(s) and extend in a substantiallyhorizontal plane projecting away from the eye(s). For this embodiment,the at least one reflective optical surface 1380 can be supported by(coupled to) a bottom edge of a forward front frame section of the HMDand angled to reflect light from the at least one image projectiondevice 1320 into the user's eye. In one embodiment, the reflectiveoptical surface 1380 is angled such that its top end is furthest fromthe user's face while its lower end is closest to the user's face. Ifdesired, the reflective optical surface can include flat (or curved)portions oriented on the side of the face.

A ray tracing analysis of a head-mounted display apparatus of the typeshown in FIG. 12 is provided in FIGS. 13 and 14. The embodiment of FIGS.13-14 uses three Fresnel lens elements 1125, 1130, and 1135, rather thanthe two Fresnel elements 1330 and 1335 of FIG. 12. In FIGS. 13 and 14,light rays 1430, 1435, and 1440 are shown such that light ray 1440enters from the environment and is combined with light ray 1430 that hasbeen reflected from reflective optical surface 1380 to create thecombined light ray 1435 that enters the user's eye when the user looksin the direction of point 1442 The user's peripheral vision capabilitiesalso allow the user to see light from other parts of reflective surface1380, distant from point 1442.

As best seen in FIG. 14, the diverging wavefront of light 1460 emanatingfrom the at least one image projection device 1320 is converged by apositive-diopter Fresnel lens system having Fresnel lenses 1125, 1130,and 1135 to provide zero diopter light 1430 which impinges on flatreflective optical surface 1380, where the light is bent into zerodiopter light 1435 that enters the pupil of the eye. This is equivalentto light coming from a point essentially infinitely distant, and thelight wavefront is flat, resulting in parallel surface normals to thewavefront, shown as rays 1435, across the entrance pupil to the eye. Thereflective optical surface 1380 also admits light 1440 from the externalenvironment (see FIG. 13), thus allowing the internal images to overlaythe external images, as also shown in FIG. 14, as externally originatinglight beams 1510.

V. Direct View HMDs

In addition to the above applications, a curved Fresnel lens system canalso be used for direct viewing of an image display system without anintervening reflective optical surface. Such a configuration will beimmersive, but can include external world information through the use ofone or more video cameras. By using a Fresnel lens system whichcomprises stacked Fresnel lenses an optical system with a short focallength and high power which can project an image of a display into awide field of view can be achieved in a compact space.

FIG. 15 is a side view representation of a user 900 wearing an immersivebinocular viewer 910 in a head-mounted display. Inside the head-mounteddisplay apparatus is at least one image display system 920 for each eyethat is adjusted for near viewing with a curved Fresnel lens system 930.An electronics package 940 may include accelerometers and/or gyroscopesto control the image that is displayed and provides location,orientation and position information to synchronize images on thedisplay with user activities. Power and video to and from the binocularviewer can be provided through a transmission cable 950 or wirelessmedium. A top view of the user 900 and viewers 910 is illustrated inFIG. 16, including eyes 955 and nose 960 in relation to the viewers 910.The Fresnel lenses of the Fresnel lens system 930 are stacked andcurved.

In this embodiment, the at least one image display system 920 is mountedto the HMD's frame directly in front of the user's eyes and orientedessentially vertically such that the pixels emanate light directly inthe direction of the user's eyes for an immersive virtual worldexperience. The curved Fresnel lens system 930 is arranged between thedisplay screen of the image display system 920 and the user's eyes andallows the eye to focus on the screen in close proximity.

The operation of the head-mounted display apparatus illustrated in FIGS.15 and 16 may be viewed using ray tracing. FIG. 17 illustrates adiverging wavefront of light 1120 emanating from the at least one imagedisplay system 920 that is collimated by a positive-diopter Fresnel lenssystem with Fresnel lenses 1125, 1130, and 1135, to provideapproximately zero-diopter light 1140 to a pupil 1145 of a user's eye.Light 1140 is equivalent to light coming from a point essentiallyinfinitely distant, and the light wavefront is flat, resulting inparallel surface normals to the wavefront, shown as rays 1140, acrossthe entrance pupil 1145 to the eye.

More particularly, in FIG. 17, it is seen that the curved Fresnel lenssystem having Fresnel lenses 1125, 1130, and 1135 allows light 1150passing through a field point 1155 at the edges of the Fresnel lenses1125, 1130, and 1135 to enter the eye from a different direction than alight beam 1160 that originates at point 1165. The curved Fresnel lenssystem with Fresnel lenses 1125, 1130, and 1135 allows the light to looklike it entered the user's field of view along a light ray path 1170.This allows an increase in the apparent field of view (the apparentangular subtense) of an amount indicated by angle 1175.

FIG. 18 is a ray tracing showing the collimated parallel rays 1140entering the eye 1205 through the pupil 1145 and being focused on thefovea 1210 where the highest acuity of vision takes place. Thesurrounding retina 1215 responds to the wider field of view but with alower acuity, for instance at points 1220 and 1225.

VI. General Considerations

In terms of the overall structure of the HMD, Table 1 sets forthrepresentative, non-limiting, examples of the parameters which a HMDdisplay constructed in accordance with the present disclosure willtypically meet. In addition, the HMD displays disclosed herein willtypically have an inter-pixel distance that is small enough to ensurethat a cogent image is established in the visual plane of the user.

Various features that can be included in the head-mounted displaysdisclosed herein include, without limitation, the following, some ofwhich have been referenced above:

(1) In some embodiments, the reflective optical surface (when used) maybe semi-transparent, allowing light to come in from the externalenvironment. The internal display-generated images can then overlay theexternal image. The two images may be aligned through the use oflocalization equipment, such as gyroscopes, cameras, and softwaremanipulation of the computer-generated imagery so that the virtualimages are at the appropriate locations in the external environment. Inparticular, a camera, accelerometer, and/or gyroscopes can be used toassist the apparatus in registering where it is in the physical realityand to superimpose its images on the outside view. In these embodiments,the balance between the relative transmittance and reflectance of thereflective optical surface can be selected to provide the user withoverlaid images with appropriate brightness characteristics. Also inthese embodiments, the real world image and the computer-generated imagecan appear to both be at approximately the same apparent distance, sothat the eye can focus on both images at once.

(2) In some embodiments, the reflective optical surface (when used) iskept as thin as possible in order minimize effects on the position orfocus of external light passing through the surface.

(3) In some embodiments, the head-mounted display apparatus provides afield of view to each eye of at least 100 degrees, at least 150 degrees,or at least 200 degrees.

(4) In some embodiments, the field of view provided by the head-mounteddisplay to each eye does not overlap the user's nose by any largedegree.

(5) In some embodiments, the reflective optical surface (when used) mayemploy a progressive transition of its optical prescription across thefield of view to maintain focus on the available display area.

(6) In some embodiments, ray tracing may be used to customize apparatusparameters for a particular implementation, such as military training,flight simulation, gaming and other commercial applications.

(7) In some embodiments, the reflective optical surface (when used)and/or the surface of the display, as well as the properties andlocations of the Fresnel lenses, and the distances between the displayand the reflective optical surface (when used) and between thereflective optical surface (when used) and the eye, can be manipulatedwith respect to a Modulation Transfer Function (MTF) specification atthe retina and/or the fovea.

(8) In some embodiments, the HMDs disclosed herein can be implemented inapplications such as, but not limited to, sniper detection, commercialtraining, military training and operations, and CAD manufacturing.

(9) Although shown as flat in the figures, the image display system mayalso have a curved light-emitting surface.

Once designed, the reflective optical surfaces disclosed herein (e.g.,the FS/UWA/RO surfaces) can be produced e.g., manufactured in quantity,using a variety of techniques and a variety of materials now known orsubsequently developed. For example, the surfaces can be made fromplastic materials which have been metalized to be suitably reflective.Polished plastic or glass materials can also be used. For “augmentedreality” applications, the reflective optical surfaces can beconstructed from a transmissive material with embedded small reflectorsthus reflecting a portion of an incident wavefront while allowingtransmission of light through the material. With specific regard to thecurved Fresnel lens systems disclosed herein, the one or more curvedFresnel lenses of those systems may be obtained already curved or madefrom curvable material, such as curvable glass or plastic to allowcurving at assembly time.

For prototype parts, an acrylic plastic (e.g., plexiglas) may be usedwith the part being formed by diamond turning. For production parts,either acrylic or polycarbonate may, for example, be used with the partbeing formed by, for example, injection molding techniques. A minimumthickness of 2 mm at the edge may be used, requiring commensuratelysized Fresnel components. A typical Fresnel facet width can be about 200microns. The reflective optical surface may be described as a detailedComputer Aided Drafting (CAD) description or as a non-uniform rationalB-Spline NURBS surface, which can be converted into a CAD description.Having a CAD file may allow the device to be made using 3-D printing,where the CAD description results in a 3D object directly, withoutrequiring machining.

The mathematical techniques discussed above can be encoded in variousprogramming environments and/or programming languages, now known orsubsequently developed. A currently preferred programming environment isthe Java language running in the Eclipse Programmer's interface. Otherprogramming environments such as Microsoft Visual C#can also be used ifdesired. Calculations can also be performed using the Mathcad platformmarketed by PTC of Needham, Mass., and/or the Matlab platform fromMathWorks, Inc., of Natick, Mass. The resulting programs can be storedon a hard drive, memory stick, CD, or similar device. The procedures canbe performed using typical desktop computing equipment available from avariety of vendors, e.g., DELL, HP, TOSHIBA, etc. Alternatively, morepowerful computing equipment can be used including “cloud” computing ifdesired.

A variety of modifications that do not depart from the scope and spiritof the invention will be evident to persons of ordinary skill in the artfrom the foregoing disclosure. The following claims are intended tocover the specific embodiments set forth herein as well asmodifications, variations, and equivalents of those embodiments.

TABLE 1 Name Description Units Minimum Maximum Distance of reflective mm10 400 surface from eye Distance of reflective mm 10 400 surface fromdisplay Display size Horizontal mm 9 100 Vertical mm 9 100 Displayresolution Horizontal pixels 640 1920+ Vertical pixels 480 1080+ HMDweight grams 1 1000  HMD size Distance in mm 10 140 front of face Humanpupil size mm 3 to 4 5 to 9 Size of reflective e.g., less than mm 30  78surface the width of the head/2 Number of reflective units 1   3+surfaces Maximum illumination e.g., bright fc, footcandles 5,00010,000   to the eye enough to allow viewing on bright sunny day Batterylife hours 3  4 Optical resolution Largest arcminute RMS 1  10 angularblur blur diameter Estimated 1  5 number of line pairs of resolutionMaximum variation in Percent 0  20 apparent brightness of the imageMaximum image Percent 0  5 distortion Estimated maximum Percent/degree 0 5 derivative of brightness

What is claimed is:
 1. A direct view head-mounted display apparatuscomprising: a frame adapted to be mounted on a head of a nominal user,the frame configured to position an image display system in front of aneye of the nominal user; and a Fresnel lens system supported by theframe and positioned between the image display system and the eye of thenominal user, the Fresnel lens system configured to project images fromthe image display system onto a wider field of view for the nominal userthan a field of view physically occupied by the image display system,the Fresnel lens system comprising: a first curved Fresnel lens that issymmetrical about an axis that extends perpendicularly from a centerpoint of the first curved Fresnel lens, wherein the first curved Fresnellens is curved along two dimensions and has a spherical shape; and asecond curved Fresnel lens that is symmetrical about an axis thatextends perpendicularly from a center point of the second curved Fresnellens, wherein the second curved Fresnel lens is curved along twodimensions and has a spherical shape, and wherein the first curvedFresnel lens and the second curved Fresnel lens are positioned in astacked relationship wherein light emitted from the image display systempasses through the first curved Fresnel lens and the second curvedFresnel lens; and wherein a light path from the image display system tothe eye of the nominal user is devoid of any reflective opticalelements.
 2. The direct view head-mounted display apparatus of claim 1wherein the Fresnel lens system has a positive-diopter optical power. 3.The direct view head-mounted display apparatus of claim 1 wherein theFresnel lens system is configured to substantially collimate lightemitted from the image display system.
 4. The direct view head-mounteddisplay apparatus of claim 1 wherein the Fresnel lens system has opticalpower, and no other optical element in the direct view head-mounteddisplay apparatus has optical power.
 5. The direct view head-mounteddisplay apparatus of claim 1 wherein the frame is configured to positionthe image display system at a distance from the eye of the nominal userthat is closer to the eye than a closest distance at which the eye iscapable of focusing.
 6. The direct view head-mounted display apparatusof claim 1 further comprising an electronics package comprising anaccelerometer and/or a gyroscope configured to measure a motion of ahead of the nominal user, wherein the electronics package is configuredto alter images displayed on the image display system to account for themotion.
 7. The direct view head-mounted display apparatus of claim 1further comprising an electronics package configured to receive imagesto be displayed on the image display system.
 8. The direct viewhead-mounted display apparatus of claim 7 wherein the electronicspackage is further configured to receive the images via a cable.
 9. Thedirect view head-mounted display apparatus of claim 7 wherein theelectronics package is further configured to receive the imageswirelessly.
 10. The direct view head-mounted display apparatus of claim1 wherein the wider field of view is immersive.
 11. The direct viewhead-mounted display apparatus of claim 1 further comprising anelectronics package configured to include external world information inimages displayed on the image display system.
 12. The direct viewhead-mounted display apparatus of claim 1 wherein the display apparatusproduces a binocular view.
 13. The direct view head-mounted displayapparatus of claim 1 wherein the Fresnel lens system consistsessentially of plastic.
 14. The direct view head-mounted displayapparatus of claim 1 wherein the Fresnel lens system further comprisesan aspheric surface.
 15. The direct view head-mounted display apparatusof claim 1 wherein the at least one curved Fresnel lens has facets withedges that lie along radial lines that pass through a point between acorner of the eye of the nominal user and a center of rotation of theeye of the nominal user.
 16. The direct view head-mounted displayapparatus of claim 1 wherein the first curved Fresnel lens has a concaveinterior portion and a convex exterior portion, the concave interiorportion having a plurality of facets configured to face the eye of thenominal user.
 17. The direct view head-mounted display apparatus ofclaim 16 wherein the convex exterior portion has a smooth, non-facetedsurface.